Hi All,
I have a fresh water cooled A4, that up until now ran great. After sitting over the winter, The engine starts fine, idles well, runs fine under load in reverse, but cuts out after 30 seconds under load in forward. I have changed the gas, replaced the fuel filter/separator, checked the tank vent (tank is new), installed new carburetor, fuel pump, ignition coil etc. I finally got it running under load on Monday after attempts at bypassing the fuel filter and even hooking up the portable tank for my outboard. I let it run under load at the dock for 30 minutes and all seemed fine. Then yesterday when I tried to put it under load in forward, it cut out again after 30 seconds. I'm out of ideas and would sure appreciate any suggestions that anyone out there might have.

I'm guessing an air/fuel mixture issue. I'm assuming - perhaps wrongly - that under load in reverse you are turning lower rpms than when in forward. Did you change the carb before you first experienced this issue or afterwards in an attempt to fix it? Wondering if your fuel mixture might be too lean/rich above lower rpms. Have you adjusted the idle mixture? Try turning it all the way in, then backing out 1 1/2 turns for starters. Turning the adjustment screw IN makes the mixture richer; turning OUT makes it leaner. Difficult to tell if you're starving for fuel or air. After it stalls, removing the flame arrestor from the carb and see if there's gas collecting there. If yes, back the idle mixture screw OUT a half turn to lean out the mixture and try again. Let the engine run a few minutes before trying again under load.

Any other recent modifications? Do you have a PCV valve? A PCV valve will require adjusting your idle mixture a little differently than without.

Several things can cause this:
1. dirt in the carburetor ... principally on the 'emulsion tube' - controls the air/gas ratio under varying engine speeds. Any amount of dirt in/on the teeny orfices of the emulsion tube can cause great shifts in fuel/air ratio.
2. Blown head gasket ... usually between 2 cylinders (3&4). Do compression check with the engine HOT and then when COLD.
3. Ignition coil is beginning to fail. .... especially common on transitorized 'points' systems where a HI TEMP coil was not used in the 'changeover' from mechanical points to transistorized.

Thank you all for the prompt replies and suggestions. Yes, I was experiencing the cutting out under load prior to replacing the carb. and new fuel filter/separator. I went at it again today by reinstalling the old fuel pump & bypassing the main fuel tank & primary filter by hooking up to my portable fuel tank for my outboard. The engine ran great under load for 30 minutes so I shut it down and hooked back up to the main fuel tank, bypassing the primary filter & instead, going through a clear plastic inline filter so I could observe the fuel flowing. The engine ran fine under load for a little over one minute and cut out. I tried this several times with the same result. I went back to running off the portable tank but now it only runs for less than a minute before cutting out even though I can see fuel flowing through the filter.
I managed to contact a mechanic who used to service A4's and he has agreed to drop by over the weekend to check things out and do a compression test among other things. Thank you all and I'll let you know how it goes.

If you have an electronic fuel pump instead of the original mechanical fuel pump this setup requires an oil pressure safety sensor (OPSS) that will cut off current to the pump if the oil pressure is not high enough. This can cause the engine to cut out.
There are 2 modes of failure you might experience with the electronic fuel pump.
1 - you might have low oil pressure and the sensor cuts off current to the fuel pump as it is designed to. This is a safety feature so you don't end up with a lot of gasoline pouring out of your carburetor.
2 - the oil pressure sensor itself can be faulty and may shut off current to the fuel pump for no apparent reason causing the engine to shut down.

You can test this by running a line from the pos. (+) side of your 12V system directly to your fuel pump. If it does not stall out when hot wired then the likely culprit is the OPSS. Beware of pumping too much gas to the carb while doing this.

If you have the mechanical fuel pump (operates only when the engine rotates) then disregard the above as it is likely some crud blocking one of the small passages in your carb. I'd suggest that you do not use your on board fuel tank as that is the likely source of crap getting stuck in your carb. Your outboard tank should have cleaner gasoline in it then a tank that has been sitting for who knows how long.
Here is what I found at the bottom of our 20 gallon on board gas tank after pumping it out. You can thank your government for subsidizing ethanol in gasoline (E10) for this phase separation issue. You can also thank the corn farmers in the midwest and their lobbies.

Hi All,
Once again,thanks for all the responses and suggestions. Believe it or not the culprit was the ignition coil which was new (3 hours of use). Yesterday, I replaced the coil and the spark plugs, which were also new and she now runs like a dream. The A4 mechanic checked the engine out today and said it was in great shape. He mentioned another common cause for the engine cutting out is a damaged low tension lead from the distributor to the coil. on the positive side, I now have a new carb., new fuel pump, new primary filter/separator, new plugs and fuel lines, etc, etc. So hopefully there will be no more mechanical problems for the rest of the cruising season.

Might be a good time to go ahead and finish what you started by replacing the fuel lines
too. Ethanol can sure chew up older fuel lines rather badly. It's one of those things that
you won't notice until a problem arises, as they dissipate on the inside instead of outside.

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